I ran the numbers, see the picture, susanfags fuck off.
On the left side, I tried to compare chuubas with close max live viewers averages (ignoring hyped special streams). Views/time is probably the best comparison metric, but only if the vtubers' average stream durations are close between each other. Then I divided those views/time by their average max viewers, so we can have them all on the same base. So polka on yt gets +93% more views/time per avg max viewers, while for flare's it's 48%, when comparing to a chuuba streaming on Twitch - Ironmouse. But of course, a part of that may be because, being hololive's fanbase more spread around the world, there may be people who were asleep/at work while she was streaming, and they had to catch the vod later.
But, fortunately, we also have a couple of minecraft streams made by Miko on Twitch, from the time when youtube was bugging so much she couldnt use it. So I took those two and compared it with a bunch of minecraft streams she's done on Youtube, going for close-ish stream duration times, results on the right of the picture. So, at least for miko's case, you can see that 35% of the live viewers are apparently being hidden on youtube's live viewers display.
I can't tell why that happens, though. A bunch of people have mentioned a bunch of different possible reasons (adblockers; queuing; spam-viewers; viewers being from different country from the streamer; a bug turned into feature; susan trying to manipulate things) - but i couldnt find definite proof for any of them. We just know it happens, we dont know the reason why.
Also Risu if you're reading this go to sleep already. yeb ehehehe